When a high accuracy level is required, dedicated trim circuitry must be provided in the integrated circuit for allowing modification of certain electrical parameters at the integrated circuit testing stage. This operation is commonly referred to as trimming.
These parameter modifications are usually performed at a wafer testing stage. Integrated circuits are fabricated on wafers of semiconductor material, and an electrical wafer sort (EWS) test is conducted on the wafer prior to severing the individual integrated circuit die destined to final packaging operations.
Trimming may even need to be carried out at the packaged circuit device final testing stage. Indeed, the isolating resin injected into the mold exerts a pressure on the semiconductor chip that may cause mechanical stress/strain on the crystalline semiconductor capable of altering electrical parameters. For this reason, it may be necessary to carry out a trimming operation when the packaged circuit device is tested.
An example of a functional circuit device that requires a trimming at a very late stage of the fabrication process is a band-gap regulator for producing a constant voltage reference insensitive to the working temperature. Regulators of this type are present in numerous system-on-chip devices, as those present in cellular telephones, data communication devices and most battery powered portable sets.
At least two low-drop linear regulators are generally provided in such apparatuses made to comply with stringent specifications, especially in connection with their operating accuracy when temperature, supply voltage, and other parameters vary. It is for these stringent reasons of ensuring accuracy of operation under varying conditions that a trimming operation should be carried out on the regulator circuit after completion of the packaging of the integrated circuit device that incorporates it.
A conventional technique, rarely implemented, includes using strips of appropriate material (commonly Cr—Si) connected between two pins of the integrated circuit device and in forcing a current through the strip to permanently modify the resistance of the strip and fix in this way the output voltage to be compensated.
According to another technique, a band-gap voltage generator is equipped with a logic circuit or a memory programmed by fuses or ZAP Zener diodes, that act/configure circuit elements (for example, MOS transistors) of the band-gap generator such to fix the generated reference voltage. This trimming technique has the advantage of being implementable with a reduced number of pins, but requires a relatively complex logic circuitry that occupies a relevant silicon area.
A conventional trimming circuit is schematically shown in FIG. 1. The trimming circuit 1 is rather simple; it is substantially a dipole comprising a first resistor R1 connected between a terminal A and a terminal B. A series of a second resistor R2 and of a ZAP Zener diode is connected in parallel to the first resistor R1.
ZAP Zener diodes behave as an open circuit until the voltage across them exceeds a predetermined threshold value, typically 7V for a component fabricated by a third-generation BCD process. When this threshold is exceeded and a relatively large current (for example 250 mA) is being forced through the diode, the ZAP Zener diode changes its electrical characteristic permanently and becomes a resistive component of few Ohms (short-circuit).
FIG. 2 shows substantially the same structure as in FIG. 1, but a fuse FUSE is provided in place of the ZAP Zener diode. The operation of the trimming circuit shown in FIG. 2 is similar to that of the circuit in FIG. 1. In this case, the fuse would only be opened (burnt) by a current larger than a predetermined value.
The approach shown in FIG. 1 is the one that is generally adopted for BCD processes, because it provides more reliable performance over time and allows the outcome of the trimming operation to be monitored by externally short-circuiting the terminals C and D before carrying out the trimming. Although quite popular, the conventional approaches of FIGS. 1 and 2 require N+1 dedicated pins for trimming N ZAP Zener diodes or fuses.
A trimming circuit adapted to be used with any kind of circuit device to be trimmed after packaging, offering the same advantages of classic trimming circuits of FIGS. 1 and 2 though requiring a reduced number of dedicated pins for carrying out the trimming step, is highly desirable.